The Anatomy of Range of Wireless Systems
Posted on Oct 19, 2012 by Frank Rayal
Questions regarding capacity, coverage, and range are the most common questions asked of me and any other wireless product vendor. To those who are intimately familiar with wireless propagation, it can be one of the most perplexing questions to answer because the answer depends on so many factors. The coverage distance of any wireless product is important for various reasons. In access networks, coverage distance factors in the number of required cell sites and therefore has a direct impact on cost. For backhaul systems, range is important to close a link between two points. This is why I want to outline some of the key aspects that determine the range of wireless systems.
This is just about the most common question asked of me and any other wireless product vendor. To those who are intimately familiar with wireless propagation, it can be one of the most perplexing questions to answer because the answer depends on so many factors. This is why I want to outline some of the key aspects that determine the range of wireless systems.
There are three fundamental aspects that determine range; system gain, the construct of the communication protocol, or more specifically, the physical and medium access control layers and the propagation environment. Let’s deal with system gain first.
System gain is the sum of all gains and losses between the transmitter and the receiver. It includes the output power, cable losses, antenna gain, and receiver sensitivity. These are all parameters related to the design of the system. Hence, a system with higher output power, higher antenna gain, and better receiver sensitivity has higher system gain. System gain is also a function of features such as the multiple antenna systems which can be used to transmit data, receive data or do both. For example, transmitting the same data stream on two antennas (that is, transmit diversity) doubles the system gain (or increase it by 3 dB). Similarly, receiving on two antennas doubles the system gain. System gain is also a function of the channel bandwidth. The wider the bandwidth, the higher the noise floor, so doubling the frequency from 20 MHz to 20 MHz, for example, reduces system gain by 3 dB.
The second impact on range comes from the construct of the communication protocol – especially the physical (PHY) as well as medium access control layers (MAC). The physical layer is the lowest layer of the communication protocol and it is responsible for transferring data between the transmitter and receiver. The PHY is typically designed for optimum performance in certain environments. When a system operates in a different environment than the one it was initially optimized for, performance is degraded and at some point the system ceases to be functional altogether. For example, orthogonal frequency division multiplex divides a wide channel bandwidth into many narrow channels using tightly packed sub-carriers. The greater number of sub-carriers in a fixed channel makes the physical layer more tolerant to multipath propagation, at the expense of greater computational processing power (and consequently higher cost). This allows the system to operate in areas where multipath creates a long power delay profile (that is the replica of a transmitted signal is received over a long time interval after the initial signal is received) as it mitigates inter-symbol interference. Power delay profile is shortest in indoor propagation due to the close proximity of obstacles to both transmitter and receiver. Thus, the power delay profile gets steadily longer in outdoor settings when moving from dense urban to open rural areas: it can be about 4-5 times as long compared to urban areas and 20-30 times as long compared to indoor settings.
The last factor is the propagation environment which most of us are familiar with. Obviously, propagation in dense urban areas with many obstacles in the path results in high attenuation while in line-of-sight conditions attenuation is significantly lower resulting in longer range.
To wrap up, provided the communication protocol is designed to allow for long range communications, the comparison between systems is based on system gain. This is where the savvy engineer would look for the right parameters to compare them on equal basis. So the next time you ask about range, be aware of what assumptions the response is based on.blog comments powered by Disqus